System.falseSenseOfSecurity

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The (in)security model [1] available to ActionScript at runtime in Macromedia Flash is absolutely worthless. The only capability that off-domain loaded movies don't have is the ability to read variables out of movie clips elsewhere in the hierarchy. This is almost entirely useless, and is really weak default behavior. If you loaded a movie, you damn well better already trust it! It could be a key logger, a denial of service attack (while (true) {}, getURL("somewhere else")), etc. If you don't trust it, DON'T LOAD IT. You have already lost if you're loading untrusted movies.

If you do trust the movie, and you end up needing the movies to communicate, you have to remember to add the System.security.allowDomain call. This means that you're writing more code for the common case, and the uncommon case is broken anyway.